Warm terrestrial planet with half the mass of Venus transiting a nearby star^★,★★

¹ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762, Porto, Portugal
e-mail: olivier.demangeon@astro.up.pt
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre, 4169-007, Porto, Portugal
³ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
⁴ Departamento de Física da Faculdade de Ciências da Universidade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal
⁵ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Sauverny, Switzerland
⁶ Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁷ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁹ Consejo Superior de Investigaciones Cientícas, Spain
¹⁰ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹¹ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹² INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹³ Fundación G. Galilei – INAF (Telescopio Nazionale Galileo), Rambla J. A. Fernández Pérez 7, 38712 Breña Baja, La Palma, Spain
¹⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
¹⁵ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁶ Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Miramare, Trieste, Italy
¹⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
¹⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748, Garching b. München, Germany
¹⁹ Department of Physics, and Institute for Research on Exoplanets, Université de Montréal, Montréal H3T 1J4, Canada

Received: 4 March 2021
Accepted: 12 July 2021

Abstract

In recent years, the advent of a new generation of radial velocity instruments has allowed us to detect planets with increasingly lower mass and to break the one Earth-mass barrier. Here we report a new milestone in this context by announcing the detection of the lowest-mass planet measured so far using radial velocities: L 98-59 b, a rocky planet with half the mass of Venus. It is part of a system composed of three known transiting terrestrial planets (planets b–d). We announce the discovery of a fourth nontransiting planet with a minimum mass of 3.06_−0.37^+0.33 M_⊕ and an orbital period of 12.796_−0.019^+0.020 days and report indications for the presence of a fifth nontransiting terrestrial planet. With a minimum mass of 2.46_−0.82^+0.66 M_⊕ and an orbital period 23.15_−0.17^+0.60 days, this planet, if confirmed, would sit in the middle of the habitable zone of the L 98-59 system. L 98-59 is a bright M dwarf located 10.6ṗc away. Positioned at the border of the continuous viewing zone of the James Webb Space Telescope, this system is destined to become a corner stone for comparative exoplanetology of terrestrial planets. The three transiting planets have transmission spectrum metrics ranging from 49 to 255, which undoubtedly makes them prime targets for an atmospheric characterization with the James Webb Space Telescope, the Hubble Space Telescope, Ariel, or ground-based facilities such as NIRPS or ESPRESSO. With an equilibrium temperature ranging from 416 to 627 K, they offer a unique opportunity to study the diversity of warm terrestrial planets without the unknowns associated with different host stars. L 98-59 b and c have densities of 3.6_−1.5^+1.4 and 4.57_−0.85^+0.77 g cm⁻³, respectively, and have very similar bulk compositions with a small iron core that represents only 12 to 14% of the total mass, and a small amount of water. However, with a density of 2.95_−0.51^+0.79 g cm⁻³ and despite a similar core mass fraction, up to 30% of the mass of L 98-59 d might be water.